530 DAVIDSON BLACK 
gent fibers from this nucleus pass ventrad and rostrad and make 
their exit from the ventro-lateral surface of the medulla on a 
level with the rostral border of the motor VII root. A second 
abducens nucleus can be made out caudal to the first, and sepa- 
rated from it by a few cell-free sections. The second abducens 
rootlet, which arises from the latter nucleus, emerges three sec- 
tions behind the caudal border of the motor VII root. The 
presence in this region of numerous diffusely arranged reticular 
elements, has made it difficult to draw a sharp line of demarca- 
tion about the limits of the abducens nuclei, and for this reason 
they are surrounded by dotted lines in figure 40 A. 
The arrangement of the abducens nuclei in Ameiurus is essen- 
tially similar to that in Solea, though the caudal abducens root 
is not separated from the emerging motor IX root by so great a 
distance in Ameiurus as in the former animal. These relations 
will be evident on comparing figure 40 A and figure 41 A (also 
figs. 31 and 33). 
It is also of interest to note that the abducens nuclei in Bartel- 
mez’ reconstruction of Ameiurus melas (1. ¢.) bear exactly the 
same sagittal relations to the motor VII and V nuclei that they 
do to these structures in Ameiurus nebulosus. 
Motor trigeminal nucleus and root (Nu. et rad. mot. N. V). In 
Solea the motor V root arises from two closely associated cell 
groups, which from their general relations may be termed re- 
spectively dorsal and ventral. The dorsal motor V nucleus occu- 
pies a position in the ventricular gray just dorsal to the fascicu- 
lus longitudinalis lateralis, and extends in this situation, from a 
short distance in front of the motor V root, to the level of its 
caudal border (figs. 35 and 40 A). Numerous large dendrites 
of the motor cells of this nucleus can be seen passing. ventrad 
between the bundles of the fasciculus longitudinalis lateralis, 
and occupying a position in the tegmentum analogous to that 
occupied by the ventral nucleus itself at more caudal levels. 
The ventral motor V nucleus begins a few sections caudad of 
the dorsal nucleus, and extends to within a short distance of the 
rostral border of the emergent motor VII root. In Solea, un- 
like Hippoglossus, it is somewhat larger than the dorsal nucleus. 
